Clock oscillators for digital devices have to work over a wide range of frequencies depending upon the application in which the digital device is being used. An oscillator is an amplifier whose output is fed back in phase to its input, causing the amplifier to oscillate. If a frequency determining element, e.g., crystal, piezoelectric resonator, inductor-capacitor tuned circuit, resistor-capacitor network, etc., is in the feedback circuit of the amplifier, the amplifier oscillation frequency will be determined by this frequency determining element. The frequency stability of the oscillator will also be determined by the frequency stability of the frequency determining element.
A problem exists however in that one oscillator/amplifier circuit design does not have the same operating characteristics over a wide range of frequencies. It is not commercially feasible to design a specific oscillator/amplifier circuit for each possible frequency of operation of the digital device. Therefore, a worst case design is generally implemented for the highest anticipated frequency of use. This design philosophy, however, creates problems at the lower frequencies of operation since the oscillator/amplifier has more gain at lower frequencies. So much more gain that at the lower operating frequencies the amplitude of the oscillation waveform may be greater than the power supply voltage rails, e.g., VDD and/or VSS. This excess oscillation amplitude beyond the power supply voltage rails causes saturation of the output circuits of the amplifier resulting in clipping (limiting) and waveform distortion of the oscillator/amplifier output signal. When the amplitude is greater than VDD and/or less than VSS, electrostatic discharge (ESD) protection circuits may be triggered causing unwanted substrate currents and hence noise. Harmonics having significant amplitudes are thereby generated which may cause higher electromagnetic interference (EMI) that may be radiated and/or conducted from the oscillator/amplifier. In addition, excessive oscillator output amplitude may cause increased heating of sensitive frequency determining circuits such as, for example but not limited to, a crystal or piezoelectric resonator. This increased heat dissipation in the frequency determining element may shorten the reliability and useful operating life thereof.